What Is Ultra-Low Frequency Voltage Withstand Test? Why Is It Used in Cable Testing?

Those who engage in electrical maintenance cannot avoid a common issue: before a cable is put into operation, it must undergo a voltage withstand test. However, for a high-voltage cable several kilometers long, with a capacitance of several hundredths of a microfarad, using traditional power frequency voltage withstand equipment for testing would require a transformer with a capacity of at least several hundred kVA, making it impractical for transportation. It is precisely to address this contradiction that ultra-low frequency voltage withstand testing technology has been widely promoted globally.

What is ultra-low frequency voltage withstand test

The Very Low Frequency (VLF) withstand voltage test is an insulation testing method that replaces the power frequency withstand voltage test by significantly reducing the test voltage frequency from 50Hz to 0.1Hz or even lower. The principle is simple: the impedance of a capacitor is inversely proportional to its frequency. The lower the frequency, the greater the capacitive impedance, and the smaller the power capacity required to drive a test sample with the same capacitance. Theoretically, at 0.1Hz, only one-five-hundredth of the power capacity required at power frequency is needed.

What issues can be identified

This method is not a makeshift approach. Numerous field practices have proven that the insulation detection effects of 0.1Hz ultra-low frequency voltage withstand test and power frequency voltage withstand test are equivalent, and both can effectively detect hidden defects such as water trees, air gaps, and interface delamination within cable insulation. Moreover, compared to DC voltage withstand test, ultra-low frequency AC testing does not accumulate space charges in XLPE insulation, making it more friendly to the tested object. This is also the fundamental reason why the national standard explicitly recommends ultra-low frequency and gradually phasing out DC voltage withstand test.

Scope of application

Ultra-low frequency voltage withstand tests are most commonly used in two scenarios: one is for cross-linked polyethylene insulated power cables of various voltage levels (handover tests and preventive tests), and the other is for verifying the stator winding insulation of large and medium-sized synchronous motors and generators. The common characteristic of these two types of equipment is their large capacitance, which happens to be the main battleground for ultra-low frequency technology.

How large should the equipment be

Taking the common 80kV ultra-low frequency high-voltage generator as an example, the controller weighs only 4 kilograms, and the booster weighs about 50 kilograms. The overall volume is much smaller than that of power frequency equipment with the same capacity, allowing it to be operated on-site in substations, rather than being used only in laboratories. This is a tangible efficiency improvement for power maintenance teams that need to travel with live equipment for preventive testing.